Role of Cr(III) deposition during the photocatalytic transformation of hexavalent chromium and citric acid over commercial TiO2 samples.

Removal of Cr(VI) and citric acid (Cit) by heterogeneous photocatalytic Cr(VI) transformation under UV light over two commercial TiO2 samples (1 g L(-1)), Evonik P25 and Hombikat UV100, was studied at pH 2 and Cr(VI) concentrations between 0.2 and 3 mM, with a fixed [Cit]0/[Cr(VI)]0 molar ratio (MR) of 2.5. In both cases, up to complete Cr(VI) removal, the temporal profiles of Cr(VI) and Cit were well adjusted to a pseudo-first order rate law with the same rate constant, evidencing that Cr(VI) removal controls the kinetics of the system. Once Cr(VI) is fully removed, Cit degradation continues with a Langmuir-Hinshelwood behaviour. In all cases, the rate constants decreased with increasing [Cr(VI)]0, and time resolved microwave conductivity (TRMC) measurements revealed that this was due to an increasing retention of Cr(III) on the surface of the photocatalysts, which reduces the lifetime of the electrons. Both kinetic experiments and TRMC measurements confirm that UV100 is not only more efficient than P25 for Cr(VI) and Cit removal, but it is also less influenced by the poisoning of the surface, consistent with its larger specific area. The use of Cit as the sacrificial agent improves the rate and efficiency of the photocatalytic Cr(VI) removal, and also the stability of the photocatalyst by preventing Cr(III) deposition, due to the formation of soluble Cr(III)-complexes, envisaged as a general result of the presence of oligocarboxylic acids in the photocatalytic Cr(VI) system.

Removal of EDTA by UV-C/hydrogen peroxide.

Mineralization of a 5 mM EDTA solution at pH 3 was evaluated via TOC removal under UV-C irradiation in the presence of H2O2 at various conditions. The highest TOC removal (78%) was obtained using a 40:1 H2O2/EDTA molar ratio, after 540 min irradiation. However, a 20:1 ratio gave slightly lower results, being economically more attractive. Best results of TOC removal were obtained under pH controlled conditions. Addition of TiO2 (1 g l(-1)) was detrimental, even in the presence of H2O2, indicating that at this concentration, TiO2 inhibits the mineralization, probably by scattering or by screening of the light.

Kinetics and mechanisms of EDTA photocatalytic degradation with TiO2.

A complete study on the photocatalytic degradation of ethylenediaminetetraacetic acid (EDTA) over TiO2 has been initiated, to establish the influence of several parameters on the reaction rate, the nature of the intermediates and the kinetic regime. TiO2 (Degussa P25) suspensions containing EDTA at pH 3 at different concentrations were irradiated under near UV light. A Langmuirian behavior was observed, from which kinetic constants have been obtained. Experiments with 5.0 mM EDTA (zero order kinetic regime) were performed for 3 hours irradiation under different conditions. Under N2 bubbling, depletion of EDTA was very low. Under O2 bubbling, the concentration of EDTA decreased around 90%. However, the corresponding decrease of TOC ranged only between 4.5% and 9%. A higher TOC reduction (22% or more) was obtained by keeping the pH constant by HClO4 addition, or by hydrogen peroxide addition. Addition of 0.5 mM Fe(III) caused a dramatic increase on the initial rate of EDTA depletion and approximately a 32% TOC decrease. Analysis of the filtered solution was performed by ion chromatography and capillary electrophoresis to monitor the disappearance of EDTA and the formation of degradation products after different irradiation times. So far, glycine, ethylenediamine, formic acid, ammonium, iminodiacetic acid, oxalic acid and glyoxylic acid have been identified.